My own empirical evidence shows, after about five years of using the same accordion bottles, that they work.

My three-year-old replenished Xtol developer, (and Edwal 12 before it), is stored in a 2liter plastic DATA Tainer bottle, and my replenisher is stored in a 2liter accordion bottle and the remaining 3liters in single 1liter bottles. When the accordion bottle is just under half full, I'll pitch another 1liter bottle full into it to fill it back up. I go perpetually like this, and I am yet to have a single issue in five years. The Xtol is stored for a long time like this, up to a year, always with full activity.

Same thing with my Ethol LPD print developer, which I also replenish. 2liter accordion bottle for the replenisher, and the rest of the gallon kit in 1liter smaller Data-tainer bottles.

What is it that makes people have bad things happen to them with the accordion bottles? I see absolutely no difference in the quality of my developers with accordion bottles compared to normal bottles.